The subject matter of the present invention is a method of preparing phthalide by the catalytic hydrogenation of phthalic acid anhydride with the aid of a nickel catalyst fixed on a support material.
The catalytic hydrogenation of phthalic acid anhydride in the presence of nickel or platinum catalysts in various solvents is known (cf. Houben-Weyl, "Methoden der organischen Chemie", 6/2, (1963), pp 732 to 733). The yields in these known methods amount to 75% at most, depending on the solvent and catalyst used. For example, when Raney nickel is used in absolute ethanol at a hydrogen gas pressure of 165 bar at 160.degree. C. reaction temperature, phthalide yields of 73% are obtained; when acetic acid butyl ester is used as solvent, the yield is around 71% when the reduction is performed at 160.degree. C. and a pressure of 130 bar. These yields are insufficient for technical processes, and also they can be achieved only when relatively pure phthalic acid anhydride is used. Furthermore, the fact that a relatively high hydrogen pressure must be used is a disadvantage of this method.
The problem therefore existed of performing the catalytic hydrogenation of phthalic acid anhydride to phthalide such that phthalide yields of more than 85% are obtained, even when the hydrogen pressure is less than 100 bar; in addition, technical phthalic acid anhydride is to be usable as starting material without substantial loss of yield.